1. Field of the Invention
The present invention relates to a sensor drive circuit for diving a sensor using the amount of variation in inductance of a coil as a sense signal.
2. Description of the Background Art
Position information of an automotive fuel pump and the like are sensed by the amount of variation in inductance of a coil. FIG. 23 is a sectional view illustrating this concept. Referring to FIG. 23, an annular coil 52 surrounds a middle portion 51A of an E-shaped iron core 51, and a copper plate movable piece 53 is disposed in the middle portion 51A. The movable piece 53 moves in rightward and leftward directions of FIG. 23 in accordance with the position indicative of the remaining quantity in the fuel pump, for example. As the movable piece 53 moves, a magnetic flux 54 developed in the iron core 51 is interrupted with varying degrees, varying the inductance of the coil 52. For instance, the inductance of the coil 52 increases as the movable piece 53 moves rightwardly in FIG. 23, and decreases as the movable piece 53 moves leftwardly.
A sensor drive circuit drives a sensor including a coil making a sensor action by using the amount of variation in inductance of the coil on the above described principle.
FIG. 24 is a block diagram illustrating the internal construction of a conventional sensor drive circuit using a coil as a sensor. Referring to FIG. 24, an oscillator circuit 1 outputs a sine-wave oscillating signal to an AGC (automatic gain control) circuit 2. The AGC circuit 2 performs automatic gain adjustment to provide an output signal of determined amplitude to a buffer 3 in response to a control signal S9 from an error amplifier 9.
An output from the buffer 3 is applied to a first end of a reference sensor 5 including a fixed coil, to first inputs of full-wave rectifier circuits 7 and 10, and to an input of an inverter 4. An output from the inverter 4 is applied to a first end of a variable sensor 6 including a variable coil and to a second input of the full-wave rectifier circuit 10.
A second end of the reference sensor 5 and a second end of the variable sensor 6 are connected at a node N1 which in turn is connected to a second input of the full-wave rectifier circuit 7.
The full-wave rectifier circuit 7 full-wave rectifies a difference signal between the signals given at the first and second inputs to output the rectified difference signal to an integration circuit 8. The integration circuit 8 integrates the output from the full-wave rectifier circuit 7 to output the integrated signal to a first input of the error amplifier 9.
The error amplifier 9 receives a reference voltage VR at its second input and outputs the control signal S9 to the AGC circuit 2 on the basis of the result of comparison between the output from the integration circuit 8 and the reference voltage VR.
The full-wave rectifier circuit 10 full-wave rectifies a difference signal between the signals given at the first and second inputs to output the rectified difference signal to an integration circuit 11. The integration circuit 11 integrates the output from the full-wave rectifier circuit 10 to output an inductance detection signal OUT to the exterior.
In such a construction, the sine-wave oscillating signal from the oscillator circuit 1 is amplified by the AGC circuit 2 and is then applied to the buffer 3. The output from the buffer 3 is applied to the first end of the reference sensor 5, and to the first end of the variable sensor 6 through the inverter 4.
Then the signals at the first and second ends of the reference sensor 5 are impressed upon the first and second inputs of the full-wave rectifier circuit 7, respectively. The signals at the first ends of the reference sensor 5 and variable sensor 6 are impressed upon the first and second inputs of the full-wave rectifier circuit 10, respectively.
The difference signal between the signals at the first and second ends of the reference sensor 5 is full-wave rectified by the full-wave rectifier circuit 7, smoothed by the integration circuit 8, and then applied to the first input of the error amplifier 9. At this time, the reference voltage VR applied to the second input of the amplifier 9 is previously set to a voltage level of a signal which, if the difference signal between the signals at the first and second ends of the reference sensor 5 is an expected sine-wave signal, is provided by full-wave rectifying and integrating the sine-wave signal.
Thus, when the output from the integration circuit 8 is lower than the reference voltage VR, the error amplifier 9 outputs to the AGC circuit 2 the control signal S9 which directs the AGC circuit 2 to increase the amplitude, and the AGC circuit 2 is controlled so that the amplitude of the output signal increases. On the other hand, when the output from the integration circuit 8 is higher than the reference voltage VR, the error amplifier 9 outputs to the AGC circuit 2 the control signal S9 which directs the AGC circuit 2 to decrease the amplitude, and the AGC circuit 2 is controlled so that the amplitude of the output signal decreases.
In this manner, the control signal S9 from the error amplifier 9 controls the output signal from the AGC circuit 2 such that the output from the integration circuit 8 becomes equal to the reference voltage VR. This permits the reference sensor 5 to provide the constant signals at the first and second ends.
The difference signal between the signals at the first ends of the reference sensor 5 and variable sensor 6 is full-wave rectified by the full-wave rectifier circuit 10 and smoothed by the integration circuit 11, and the inductance detection signal OUT is outputted to the exterior.
At this time, the amplitude (A5) of the difference signal between the signals at the first and second ends of the reference sensor 5 is controlled to be held constant by the AGC circuit 2. Thus, the relation between the amplitude (A5) and the amplitude (A56) of the difference signal between the signals at the respective first ends of the reference sensor 5 and variable sensor 6 is represented by the relational expression using the inductance L0 of the reference sensor 5 and the inductance L1 of the variable sensor 6. ##EQU1##
That is, the reference amplitude A5 plus (L1/L0).multidot.A5 equals the amplitude A56. Therefore, verification of the voltage of the inductance detection signal OUT allows detection of the amount of variation in inductance L1 by the inverse operation.
FIG. 25 illustrates the operation of the full-wave rectifier circuit 7 (10) and the integration circuit 8 (11). Upon receipt of a difference signal SS1, the full-wave rectifier circuit 7 rectifies the difference signal SS1 to a signal SS2 which in turn is smoothed by the integration circuit 8 into a signal SS3. When the waveform of the difference signal SS1 is expressed by A sin .theta.+v1, the waveform of the smoothed signal SS3 is expressed as (2A/.pi.)+v1.
Accordingly, the reference voltage VR is set to (2A'/.pi.)+(v1)' when the ideal waveform of the difference signal SS1 is expressed by A' sin.theta.+(v1)'.
In the conventional sensor drive circuit as above constructed, the amplitude of the output signal from the AGC circuit 2 has been controlled on the basis of the result of comparison between the reference voltage VR and the voltage level provided by full-wave rectifying and smoothing the amplitude of the difference signal between the signals at the first and second ends of the reference sensor 5.
Thus, the verification voltage applied to the error amplifier 9 is a voltage compressed to 2/.pi. times the amplitude of the difference signal impressed upon the full-wave rectifier circuit 7, resulting in less accurate control by the AGC circuit 2.
The result is inaccurate constant amplitude control of the signals at the first and second ends of the reference sensor 5, causing the problem of less accurate verification of the inductance L1 of the variable sensor 6.
Further, the voltage level of the inductance detection signal OUT from the integration circuit 11 is also compressed to 2/.pi. times the amplitude of the difference signal impressed upon the full-wave rectifier circuit 10. From this viewpoint, the problem arises that the amount of variation in inductance L1 of the variable sensor 6 is detected with lower accuracy in response to the inductance detection signal OUT.